1. Field of the Invention
The present invention relates to a lever type hoist, and more particularly to a lever type hoist equipped with a reverse rotation preventive mechanism when a load is applied to a load sheave, and capable of idling the load sheave in order to adjust the position of a lower hook in no-load state.
2. Description of the Related Art
An example of the structure of a conventional lever type hoist is shown in FIG. 27. This lever type hoist mainly comprises a drive shaft 101, a pressure bearing member 108, a reverse rotation preventive wheel 105, a pressing drive member 108, a spring 109, a rotation limiting member 110, and an operation wheel 111. The pressure bearing member 103 is driven into the innermost side (the left side in the drawing) of a threaded part 102 of the drive shaft 101. The reverse rotation preventive wheel 105 is interposed between friction members 106, 106, and the reverse rotation preventive wheel 105 and the friction members 106 are rotatably fitted on the outer circumference of a boss part 104 of the pressure bearing member 103. The pressing drive member 108 is screwed into the drive shaft 101, and is moved back and forth along the threaded part 102 of the drive shaft 101 by manipulation of an operation lever 107. Between the pressure bearing member 103 and pressing drive member 108, a spring 109 is placed, and the spring 109 is thrust in a direction of detaching the pressure bearing member 103 and pressing drive member 108 from each other.
In the part of the drive shaft 101 projecting to the outer side in the axial direction (the right side in the drawing) from the pressing drive member 108, a rotation limiting member 110 is spline-coupled, while the operation wheel 111 is rotatably engaged with the outer circumference of the rotation limiting member 110. At the inner end of the rotation limiting member 110, a rotation limiting projection 113 is formed, and a pressing release projection 112 is formed at the inner end of the operation wheel 111. At the outer end of the pressing drive member 108, projections extending in the radial direction are formed. As the rotation limiting projection 113 protrudes among the projections of the pressing drive member 108, the angle of the pressing drive member 108 rotating about the drive shaft 101 is limited. When the operation wheel 111 is rotated counterclockwise as seen from the right side in the drawing, the pressing release projection 112 abuts against the protrusions of the pressing drive member, and the pressing drive member 108 is also rotated counterclockwise.
In the lever type hoist, incidentally, when a load is suspended on a lower hook attached to a load chain, it is rotated in the direction of the load sheave being pulled down by this load (in the counterclockwise direction). The load sheave works to rotate the drive shaft 101 in the same direction through a gear train. At this time, the pressing drive member 108 is stopped by the operation lever 107, and is prevented from rotating together with the drive shaft 101. Therefore, when the drive shaft 101 is put in rotation, the pressing drive member 108 screwed in the drive shaft 101 is moved toward the friction members 106 by resisting the pressing force of the spring 109, and presses the friction members 106, and rotation of the drive shaft 101 is arrested by the frictional force at this time.
When the load on the lower hook is large, by rotating the pressing drive member 108 in the clockwise direction, the reverse rotation preventive wheel 105 is forcefully pushed in between the pressing drive member 108 and pressure bearing member 103, and therefore the pressing state by the pressing drive member 108 will not be loosened during reciprocal rotation of the operation lever 107. In the lever type hoist shown in FIG. 27, however, since the spring 109 is placed between the pressure bearing member 103 and pressing drive member 108, and when the load suspended on the lower hook is small, the force of the spring 109 acting to detach the pressing drive member 108 from the friction members 106 may be greater than the force of moving the pressing drive member 108 toward the friction members 106. In such a case, if the pressing member 108 rotates in the clockwise direction, the reverse rotation preventive wheel 105 may not be held with a sufficiently strong force. When the operation lever is moved reciprocally in such a state, is the operation lever is turned in the counterclockwise direction, the small load suspended on the lower hook is moved downward by its own gravity, which induces a risk of not only damaging a colliding object during move, but also injuring the worker.
The invention is devised in the light of such problems, and it is hence a primary object of the invention to present a lever type hoist capable of preventing rotation of the drive shaft even when the load applied on the load sheave is smaller than a specific weight by keeping the pressing drive member in tight contact with the friction members, so that the small load may not move downward by its own weight. It is another object thereof to present a lever type hoist capable of rotating continuously and lightly when adjusting the position of the lower hook in no-load state.